Heatsink and fabrication method thereof

ABSTRACT

A heatsink includes a substrate of a sintered compact including Cu and W, and a thin diamond film layer formed on the surface of the substrate with good adherence. The Cu content in the substrate is at least 5% by weight. In an X-ray diffraction chart obtained by irradiating the thin diamond film layer with an X-ray, the diffraction peak intensity of the (110) plane of W is at least 100 times the diffraction peak intensity of the (200) plane of Cu. The heat sink is fabricated by immersing the substrate in acid to reduce the Cu content of a surface region thereof and to roughen exposed W at that surface region, and then forming the thin diamond film layer on that surface region by vapor synthesis. Alternatively, a thin diamond film layer is formed on a surface of a porous body substrate, and then a hole in the porous body substrate is filled with Cu.

GROSS REFERENCE TO RELATED APPLICATION

[0001] This application is a Divisional of our copending U.S.application Ser. No. 09/232,011 filed Jan. 14, 1999, now allowed.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a heatsink and a fabricationmethod thereof. Particularly, the present invention relates to aheatsink on which is mounted a semiconductor element of relatively greatheat generation such as a laser diode, a CPU (central processing unit),a MPU (microprocessor unit), a high frequency amplifier device, and thelike, having a multilayer structure of a diamond layer and a metallayer, and a method of fabricating such a heatsink.

[0004] 2. Description of the Background Art

[0005] The aforementioned high power semiconductor devices generate agreat, amount of heat during operation. The heat generated by thesesemiconductor elements has become greater in accordance with improvementin the output and the operating frequency. The need of compact andlight-weight electronic equipment is great in the industry while thepackaging density of the semiconductor element is continuouslyincreasing. The increase in the heat generation and packaging density ofsemiconductor elements implies more stringent requirements with respectto the heat radiation characteristic of the heatsink employed in modulesin which high power semiconductor elements are mounted.

[0006] Regarding such heatsinks that require great heat dissipation, asemiconductor element is mounted on a heatsink formed of a material ofhigh thermal conductivity to prevent the semiconductor element frombecoming too hot. For heatsinks that incorporate high powersemiconductor elements such as a high power transistor or microwavemonothylic IC (MMIC) of great heat generation, beryllium oxide (BeO),for example, superior in thermal conductivity and dielectric property isconventionally used widely.

[0007] Diamond is known as the substance having the highest thermalconductivity. Research has been effected to apply diamond to theheatsink that is used for incorporating a semiconductor element.

[0008] As a heatsink employing diamond, development is in progress of aheatsink formed entirely of diamond, and a heatsink having a diamondfilm formed on a metal substrate.

[0009] Since natural diamond is precious and artificial diamond iscostly, the cost of the heatsink will increase if the amount of diamondtherein becomes greater. Therefore, a heatsink formed entirely ofdiamond is used with respect to a semiconductor element of high heatgeneration such as a high power laser only in the application where heatradiation is so insufficient that it prevents exhibition of properperformance when a substitute is used or in the application such asduring the stage of research where the cost is not yet estimated. Aheatsink having a diamond film formed on a metal substrate is used inproducts that must have the cost reduced.

[0010] By using a heatsink formed partially of metal, the cost can bedecreased although the thermal conductivity is degraded in comparison toa heatsink formed only of diamond. Therefore, the cost and performanceof the heatsink is substantially proportional. It can be said that aheatsink of higher thermal conductivity becomes more expensive.Therefore, there is a demand for an economical heatsink of high thermalconductivity.

[0011] In response to such demands, a heatsink of a multilayeredstructure with a thin diamond film formed on a metal of favorablethermal conductivity is disclosed in, for example, Japanese PatentLaying-Open No. 5-326767.

[0012] Conventionally, BeO superior in thermal conductivity has beenwidely used for the heatsink. However, the level of the heat radiationcharacteristic that is currently required has become so high that evenBeO is even not sufficient. An approach has been made to reduce thethickness of the BeO substrate to reduce thermal resistance. However, itis difficult to process BeO per se. Furthermore, BeO is toxic. It can besaid that reduction in the thickness has come to its limit.

[0013] As to the heatsink disclosed in the above publication, copper andcopper-tungsten alloy which are metals of favorable thermal conductivityare mentioned as the substance of the substrate. These materials aresuitable for the heatsink since the thermal conductivity thereof is highcomparable to other metal materials the cost is relatively low.

[0014] However, there was problem that it is difficult to grow a thindiamond film on a substrate that includes copper in favorable adherencesince the copper in the substrate does not produce carbide, does notabsorb carbon, and is not occluded with carbon, as described in NewDiamond, Vol. 10, No. 3 (34), pp. 26 and 27.

[0015] Copper has a high thermal expansion coefficient whereas diamondhas a low thermal expansion coefficient. Therefore, there is a problemthat the thin diamond film will peel off the substrate as thetemperature of the heatsink becomes higher due to the difference inthermal expansion coefficient between copper and diamond.

[0016] If the difference in thermal expansion between the substrate andthe diamond is small, warping in the diamond heatsink will not occur.Only stress will be generated within the thin diamond film even when theheatsink attains high temperature. However, the thermal expansion ofcopper or a sintered compact including copper is greater than that ofdiamond, resulting in the problem of warping in the heatsink.

SUMMARY OF THE INVENTION

[0017] In view of the foregoing, an object of the present invention isto provide a heatsink that can have a thin diamond film formed in goodadherence on a substrate of favorable thermal conductivity.

[0018] Another object of the present invention is to provide a heatsinkthat can have occurrence of warping suppressed.

[0019] According to an aspect of the present invention, a heatsinkincludes a substrate of a sintered compact including Cu and W, and athin diamond film layer formed on the surface of the substrate. The Cucontent in the substrate is at least 5% by weight. In an X-raydiffraction chart obtained by irradiating a thin diamond film layer withan X-ray, the diffraction peak intensity of the (110) plane of W is atleast 100 times the diffraction peak intensity of the (200) plane of Cu.

[0020] In such a heatsink, the amount of W at the surface of thesubstrate is relatively great whereas the amount of Cu at the surface ofthe substrate becomes relatively smaller. Therefore, the adherencebetween the substrate and the thin diamond film layer formed on thesurface of the substrate is improved. As a result, the heat locallygenerated from the semiconductor element mounted on the thin diamondfilm layer can be promptly dissipated at the in-plane of the thindiamond film layer by virtue of the effect of the thin diamond filmlayer as a heat spreader (effect of heat dissipation) to be conveyed tothe substrate. The thermal conductivity of the substrate is increasedsince the Cu content in the substrate is at least 5% by weight.

[0021] In an X-ray diffraction chart obtained by irradiating the thindiamond film layer with an X-ray, it is preferable that a peak of WC(tungsten carbide) appears. In this case, the adherence between the thindiamond film layer and the substrate is improved.

[0022] According to another aspect of the present invention, a heatsinkincludes a substrate of a sintered compact including Cu and W, and athin diamond film layer formed on the surface of the substrate. The Cucontent in the substrate is at least 5% by weight. In an X-raydiffraction chart obtained by irradiating the thin diamond film layerwith an X-ray, the diffraction peak intensity of the (211) plane of W isat least 30 times the diffraction peak intensity of the (200) plane ofthe Cu.

[0023] In such a heatsink, the amount of W at the surface of thesubstrate becomes relatively greater whereas the amount of Cu at thesurface of the substrate becomes relatively smaller. Therefore, theadherence between the substrate and the thin diamond film layer formedon the surface of the substrate is improved. As a result, the heatlocally generated from the semiconductor element mounted on the thindiamond film layer is rapidly dissipated at the in-plane of the thindiamond film layer to be subsequently conveyed to the substrate byvirtue of the effect of the thin diamond film layer as a heat spreader(effect of heat dissipation). Also, the thermal conductivity of thesubstrate becomes higher since the Cu content in the substrate is atleast 5% by weight.

[0024] In an X-ray diffraction chart obtained by irradiating the thindiamond film layer with an X-ray, it is preferable that a peak of WC(tungsten carbide) appears. In this case, the adherence between the thindiamond film layer and the substrate is improved.

[0025] According to a further aspect of the present invention, aheatsink includes a substrate including Cu and a metal of a low thermalexpansion coefficient, and a thin diamond film layer formed on thesurface of the substrate. The Cu content in the substrate is at least 5%by weight. The Cu content in the substrate becomes lower as a functionof approaching the surface of the substrate.

[0026] In the heatsink of the above structure, the Cu content at thesurface of the substrate is lowest. Therefore, the adherence between thesubstrate and the thin diamond film layer on the substrate is improvedsince the amount of Cu that does not easily attach to carbon is small atthe surface of the substrate. As a result, the heat locally generatedfrom the semiconductor element is rapidly dissipated at the in-plane ofthe thin diamond film layer to be subsequently conveyed to the substrateby virtue of the thin diamond film layer serving as a heatsink spreader(effect of heat dissipation). Also, the thermal conductivity of thesubstrate is increased since the Cu content in the substrate is at least5% by weight.

[0027] The Cu content at a region of the substrate that is not more than10 μm in depth from the surface of the substrate is preferably not morethan 50% of the entire Cu content of the substrate. By adjusting thecontent amount of Cu at the surface of the substrate, adherence betweenthe thin diamond film layer and the substrate is improved. If the Cucontent at a region that is not more than 10 μm in depth from thesurface of the substrate exceeds 50% of the entire Cu content of thesubstrate, the rate of presence of Cu becomes so high that the thindiamond film layer is easily peeled off the substrate. By setting the Cucontent at the surface of the substrate to be less than 50% of theentire Cu content of the substrate, warping in the substrate can besuppressed due to an appropriate amount of Cu remaining in thesubstrate.

[0028] The substrate is preferably a Cu—W sintered compact or a Cu—W—Mosintered compact The Cu—W sintered compact or the Cu—W—Mo sinteredcompact must have a thermal conductivity of at least 100 W/m·K in orderto exhibit the advantage of the present invention.

[0029] W particles are exposed at the surface of the substrate. Thesurface roughness R_(Z) of the W particle is preferably at least 0.05μm. The diamond nucleus is easily generated from the convex portion ofthe W particle. By setting the surface roughness R_(Z) of the W particleas above, the nucleus generation density of diamond is improved.Therefore, the number of contact points between the substrate and thethin diamond film layer is increased. Thus, adherence between the thindiamond film layer and the substrate can further be improved.

[0030] If the surface roughness R_(Z) of the W particle is less than0.05 μm, the nucleus generation density is reduced since the convexportion in the W particle is reduced. This means that the adherencebetween the thin diamond film layer and the substrate is degraded sothat the thin diamond film layer easily peels off the substrate.

[0031] Preferably, an intermediate layer in which the Cu content isapproximately 0% by weight is formed between the surface of thesubstrate and the thin diamond film layer. By provision of anintermediate layer that does not include Cu between the substrate andthe thin diamond film layer, the thin diamond film layer will not comeinto contact with the Cu in the substrate. Therefore, adherence betweenthe thin diamond film layer and the substrate is further improved.

[0032] The thickness of the substrate is preferably at least 200 μm andnot more than 10000 μm. In order to maintain the strength as asubstrate, the thickness of the substrate is preferably at least 200 μm.In order to avoid the thermal resistance of the heatsink from becomingtoo great, the substrate thickness is preferably not more than 10000 μm.

[0033] The thickness of the thin diamond film layer is preferably atleast 10 μm. In this case, the thin diamond film layer functions todiffuse in-plane the heat generated by the semiconductor element toprevent the heat from being partially confined. The thickness of thethin diamond film layer must be at least 10 μm for this function.

[0034] The thermal conductivity of a thin diamond film layer isgenerally in the range of 500 W/m·K−2000 W/m·K depending upon thequality of the diamond. In the present invention, the thermalconductivity of the thin diamond film layer must be at least 700 W/m·Kin order to exhibit the effect of the present invention.

[0035] Preferably, the substrate has a thermal conductivity of at least100 W/m·K, and a thickness of at least 200 μm and not more than 700 μm,whereas the thin diamond film layer has a thickness of at least 10 μmand not more than 200 μm. Here, the thin diamond film layer expandsanalogous to the substrate since it is very thin. Therefore, theexpansion of the thin diamond film layer and that of the semiconductorelement provided thereon is substantially equal. Accordingly, crackingin the semiconductor element can be suppressed.

[0036] Furthermore, the thermal conductivity of the thin diamond filmlayer is preferably at least 1000 W/m·K.

[0037] A method of fabricating a heatsink according to the presentinvention includes the steps of reducing the Cu content at the surfacelayer region of the substrate by immersing the surface of the substrateincluding Cu and a metal of a low thermal expansion coefficient in acid,and roughening the surface of the exposed metal of a low thermalexpansion coefficient, and forming by vapor synthesis a thin diamondfilm layer on the surface of the substrate subjected to acid treatment.

[0038] According to the heatsink fabrication method including the abovesteps, the Cu content at the surface of the substrate is reduced bysubjecting the surface of the substrate to acid treatment. Then, a thindiamond film layer is formed on the surface. Therefore, a thin diamondfilm layer can be formed on the substrate in good adherence.

[0039] The step of reducing the Cu content at the surface layer regionof the substrate includes roughening the exposed surface of the metal ofa low thermal expansion coefficient. Therefore, adherence between thethin diamond film layer and the substrate is further improved since thethin diamond film is formed on the roughened metal of low thermalexpansion coefficient.

[0040] The acid is preferably a solution selected from the groupconsisting of hydrochloric acid, nitric acid, sulfuric acid,hydrofluoric acid (HF), hydrogen peroxide (H₂O₂) and chromic acid, or amixed solution thereof. By using these acids, the surface of thesubstrate can be appropriately roughened to facilitate formation of thethin diamond film layer.

[0041] The step of reducing the Cu content at the surface layer regionof the substrate preferably includes a first acid treatment step ofimmersing the surface of the substrate in a certain acid, and a secondacid treatment step of immersing the substrate subjected to the firstacid treatment in an acid different from the certain acid.

[0042] In forming a diamond film by vapor synthesis, a diamond nucleusis generated at a deep location from the surface of the substrate. Inother words, the root of the thin diamond film is present at a deepregion in the substrate, so that the anchor effect can be expected.

[0043] The substrate is preferably at least one type of a sinteredcompact selected from the group consisting of a Cu—W sintered compact, aCu—Mo sintered compact and a Cu—W—Mo sintered compact.

[0044] W particles are exposed at the surface of the substrate immersedin acid. The surface roughness R_(Z) of the W particle is preferably atleast 0.05 μm. By such a surface roughness, small particles of diamondpermeate into the surface of the W particles. A thin diamond film layeris grown with the diamond particle as the nucleus. Thus, adherencebetween the thin diamond film layer and the substrate is improved.

[0045] The acid treatment that reduces the Cu content is preferablycarried out until the peak of Cu is not detected in an X-ray diffractionchart obtained by irradiating the surface of the substrate with anX-ray.

[0046] In this case, the Cu content at the surface of the substrate issufficiently reduced. Therefore, adherence between the substrate and thethin diamond film layer formed thereon is improved.

[0047] The acid treatment of reducing the Cu content is preferablycarried out until the porosity of the region of the substrate that iswithin 30 μm in depth from the surface of the substrate is at least 5%by volume and not more than 70% by volume, and the Cu content at theregion within 30 μm in depth from the surface of the substrate is notmore than 50% of the entire Cu content of the substrate.

[0048] By setting the porosity to the above described range, the grainsof diamond can easily permeate into the hole to facilitate nucleusgeneration.

[0049] If the porosity is less than 5% by volume, nucleus generationcannot easily occur. If the porosity is greater than 70% by volume, theholes become so great that the thermal conductance is reduced. As aresult, the property as a heatsink will be degraded.

[0050] Further preferably, the porosity at the region within 30 μm indepth from the surface of the substrate is at least 10% by volume andnot more than 50% by volume.

[0051] By setting the Cu content at the region of the substrate within30 μm in depth from the surface of the substrate to be less than 50% ofthe entire Cu content of the substrate, warping of this heatsink willnot easily occur.

[0052] Preferably, a step of carrying out a process of scratching thesurface of the substrate is included prior to formation of the thindiamond film layer. In this case, diamond is easily nucleated from thescratch at the surface of the substrate. Many diamond nuclei can begenerated at the surface of the substrate to promote the speed in thegrowth of the thin diamond film layer. Also, the thickness of the thindiamond film layer can be made uniform.

[0053] The scratching process preferably includes the step of scratchingthe surface of the substrate using diamond. In this case, diamond isattached to the surface of the substrate in the scratching step of thesurface of the substrate to become the nucleus in forming the thindiamond film layer. Therefore, growth of the thin diamond film layer canfurther be promoted.

[0054] Vapor synthesis is used in forming a thin diamond film layer onthe substrate. This vapor synthesis includes hot filament CVD (chemicalvapor deposition), plasma CVD, flame method, and the like.

[0055] According to still another aspect of the present invention, aheatsink includes a substrate, and a thin diamond film layer formed onthe substrate. The substrate includes a porous body of a low thermalexpansion coefficient, and Cu filled in the hole of the porous body. Atthe surface layer of the substrate, a thin diamond film layer is presentin the hole. The diamond film layer is permeating into the hole at asurface layer of the porous body.

[0056] In the heatsink of the above structure, the difference in thermalexpansion coefficient between the substrate and the thin diamond filmlayer formed on the substrate is small since the porous body forming thesubstrate has a low thermal expansion coefficient. Adherence between thesubstrate and the thin diamond film layer is improved since the thindiamond film layer is present at the surface layer of the porous body.As a result, the thin diamond film layer can be prevented from peelingoff the substrate even when the heatsink attains high temperature.

[0057] Preferably, the substrate has a thermal conductivity of at least100 W/m·K and a thickness of at least 200 μm and not more than 700 μmwhereas the thin diamond film layer has a thickness of at least 10 μmand not more than 200 μm. The thin diamond film layer expandssubstantially analogous to the substrate due to its small thickness.Therefore, the expansion of the thin diamond film layer is substantiallyequal to that of the semiconductor element provided thereon. As aresult, cracking in the semiconductor element can be suppressed.

[0058] Furthermore, the thermal conductivity of the thin diamond filmlayer is preferably at least 1000 W/m·K.

[0059] The porous body is preferably at least one type of a sinteredcompact selected from the group consisting of a W sintered compact, a Mosintered compact, and a W—Mo sintered compact.

[0060] Furthermore, the porosity of the porous body is preferably atleast 15% by volume and not more than 60% by volume. If the porosity ofthe porous body is less than 15% by volume, the heat conductivity isreduced when the hole is filled with Cu. If the porosity exceeds 60% byvolume, the thickness of the thin diamond film layer will becomenonuniform.

[0061] A heatsink fabricating method according to the present inventionincludes the steps of forming a thin diamond film layer on the surfaceof a porous body having a low thermal expansion coefficient, and flingthe hole of the porous body with Cu after formation of the thin diamondfilm layer.

[0062] According to the heatsink fabrication method including the abovesteps, a diamond nucleus is generated from the surface of the porousbody in forming a thin diamond film layer. Therefore, the nucleus of thethin diamond film layer is generated from a deep portion remote from thesurface of the porous body. In other words, the base of the thin diamondfilm layer is present at a deep region remote from the surface of thesubstrate. Therefore, the anchor effect can be expected.

[0063] Since a substrate of a porous body with a low thermal expansioncoefficient is used, the amount of thermal expansion is small when athin diamond film layer is grown. Since the amount of thermal expansionis small, warping that occurs by the difference in thermal expansionbetween the thin diamond film layer and the porous body can besuppressed during the stage of cooling the porous body after filmgrowth.

[0064] Since Cu fills the hole of the porous body, the hole of theporous body is exactly filled with Cu that has favorable thermalconductivity. As a result, the thermal conductance of the substrate isimproved. Accordingly, the thermal conductance of the entire heatsink isimproved.

[0065] Since the porous body is absent of Cu during the stage of forminga thin diamond film layer on the porous body, the thin diamond filmlayer can be formed in favorable adherence on the porous body.

[0066] The step of forming a thin diamond film layer preferably includesthe step of forming a thin diamond film layer on the surface of theporous body by vapor synthesis. Here, the vapor synthesis methodincludes the hot filament CVD (chemical vapor deposition), plasma CVD,flame method, and the like.

[0067] The porous body is preferably at least one type of a sinteredcompact selected from the group consisting of a W sintered compact, a Mosintered compact, and W—Mo sintered compact.

[0068] The porosity of the porous body is preferably at least 15% byvolume and not more than 60% by volume. By setting the porosity to theabove range, generation of a diamond nucleus from the surface of theporous body is facilitated. The thermal conductivity when Cu is filledis increased to improve the thermal conductivity of the entire sink.

[0069] If the porosity is less than 15% by volume, a diamond nucleuscannot be easily generated from a deep region in the porous body. As aresult, adherence between the porous body and the thin diamond filmlayer is degraded. Furthermore, the thermal conductance of the heatsinkis degraded since the amount of Cu in the hole is reduced.

[0070] If the porosity exceeds 60% by volume, it will become difficultto form a thin diamond film layer of uniform thickness on the surface ofthe porous body although a diamond nucleus can be generated at a deepregion of the porous body. Furthermore, although the generation densityof the diamond nucleus is reduced and the grain of the diamond formingthe thin diamond film layer is increased, the surface roughness of thethin diamond film layer becomes greater. Accordingly, there is a problemthat the thickness of the thin diamond film layer is not uniform, andpolishing the thin diamond film layer is time consuming.

[0071] The step of filling the hole of the porous body with Cupreferably includes the step of permeating molten Cu into the hole ofthe porous body.

[0072] The step of filling the hole of the porous body with Cupreferably includes the step of heating and melting Cu and permeatingmolten Cu in the hole after the porous body is placed on solid Cu.

[0073] By placing solid Cu on a heating device such as a heater,arranging the porous body thereon with a thin diamond film layer at thetop surface and the Cu melted, the Cu will permeate into the porous bodyby the capillary action. According to this method, arrangement betweenCu and the porous body is feasible. Furthermore, since the molten Cu canbe suppressed from spattering around, adherence of contaminants on thesurface of the thin diamond film layer can be prevented.

[0074] The step of filling the hole of the porous body with Cupreferably includes the step of heating and melting Cu to permeate themolten Cu into the bole after solid Cu is placed on the porous bodywhere the thin diamond film layer is formed.

[0075] Since the solid Cu placed on the porous body is melted, the Cupermeates into the porous body by the weight and capillary action of theCu. Therefore, the charging rate of Cu becomes faster.

[0076] The step of filling the hole of the porous body with Cupreferably includes the step of storing molten Cu in a container andimmersing the porous body with the formed thin diamond film layer in themolten Cu to permeate the melted Cu into the hole.

[0077] Since the porous body is immersed in the molten Cu, Cu permeatesequally from all the faces of the porous body except for the face wherethe thin diamond film layer is formed. Also, the permeation speedbecomes faster.

[0078] Preferably, the step of scratching the surface of the porous bodyprior to formation of a thin diamond film layer is included. Since adiamond nucleus is easily generated from the scratch, many diamondnucleus are generated at the surface of the porous body. A thin diamondfilm layer is grown faster and with uniform thickness.

[0079] The scratching process preferably includes the step of scratchingthe surface of the porous body using diamond. Diamond is attached to thesurface of the porous body during the stage of scratching the surface ofthe porous body. This diamond becomes the nucleus in forming a thindiamond film layer. Accordingly, the growth of the thin diamond filmlayer is facilitated.

[0080] The foregoing and other objects, features, aspects and advantagesof the present invention will become more apparent from the followingdetailed description of the present invention when taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0081]FIG. 1 is a schematic diagram of a hot filament CVD apparatus fordiamond vapor synthesis employed in the present invention.

[0082]FIG. 2 is an X-ray diffraction chart obtained by irradiating asubstrate prior to being immersed in acid with an X-ray.

[0083]FIG. 3 is a graph showing the concentration of Cu, W and C in thesubstrate prior to immersion in acid.

[0084]FIG. 4 is a graph showing the concentration of Cu, W and C in asubstrate after immersion in acid and before formation of a thin diamondfilm layer.

[0085]FIG. 5 is an X-ray diffraction chart obtained by irradiating athin diamond film layer after formation with an X-ray.

[0086]FIG. 6 is a graph showing the concentration of Cu, W and C in asubstrate after formation of a thin diamond film layer and in the thindiamond film layer.

[0087]FIG. 7 is a schematic diagram of a microwave plasma CVD apparatusfor diamond vapor synthesis employed in the present invention.

[0088]FIG. 8 is an X-ray diffraction chart obtained by irradiating thethin diamond film layer obtained in Example 3 with an X-ray.

[0089]FIG. 9 is a scanning type electron microphotograph of a certainportion of a sample obtained by Example 3.

[0090]FIG. 10 is a scanning type electron microphotograph of anotherportion of a sample obtained by Example 3.

[0091]FIG. 11 is a scanning type electron microphotograph of a portionof a substrate subjected to acid treatment according to Example 11.

[0092]FIG. 12 is a scanning type electron microphotograph of anotherportion of a substrate subjected to acid treatment according to Example11.

[0093]FIG. 13 is an X-ray diffraction chart of a substrate subsequent toacid treatment according to Example 15.

[0094]FIG. 14 is an X-ray diffraction chart of a substrate subsequent toacid treatment according to Comparative Example 3.

[0095]FIG. 15 shows the step of a heatsink fabricating method accordingto Example 1 of the present invention.

[0096]FIG. 16 is a schematic diagram of a thin diamond film layerproduced according to Example 1 of the present invention.

[0097]FIG. 17 is a schematic diagram of a thin diamond film layer formedon an intermediate layer, produced according to Example 2 of the presentinvention.

[0098]FIGS. 18, 19 and 20 are schematic diagrams showing the first,second, and third steps, respectively, of a heatsink fabricating methodof the present invention.

[0099]FIG. 21 is a sectional view schematically showing a semiconductormodule according to Example 19 of the present invention.

[0100]FIG. 22 is a sectional view schematically showing a semiconductormodule according to Example 20 of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

[0101] Referring to FIG. 1, a hot filament CVD (chemical vapordeposition) apparatus for diamond vapor synthesis employed in thepresent invention includes a reactor 21, a gas inlet 22, gas outlet 23,an AC power supply 24, a tungsten filament 25, a substrate holder 27, acooling water inlet 28, and a cooling water outlet 29.

[0102] Reactor 21 includes gas inlet 22 through which material gas isintroduced and gas outlet 23 from which the material gas or gasgenerated from the material gas is output.

[0103] Tungsten filament 25 is provided in reactor 21. Tungsten filament25 is connected to AC power supply 24. Tungsten filament 25 is broughtto red heat by conducting current from AC power supply 24 to tungstenfilament 25.

[0104] A molybdenum substrate holder 27 is provided beneath tungstenfilament 25 to hold a substrate. Since tungsten filament 25 attains ahigh temperature by being brought to red heat, this substrate holder 27is also increased in temperature. Substrate holder 27 is provided withcooling water inlet 28 through which cooling water to cool substrateholder 27 is input and cooling water outlet 29 from which the coolingwater is output.

[0105] A substrate formed of a Cu—W sintered compact having the Cucontent of 11% by weight and the size of 13.5 mm×13.5 mm×0.635 mm(lengthwise×breadthwise×thickness) was prepared. An X-ray of a CuKαvessel was directed to the surface of the substrate to obtain an X-raydiffraction chart. The result is shown in FIG. 2.

[0106] It is appreciated from FIG. 2 that the peak arising from W(tungsten) is greater than the peak arising from Cu (copper). It istherefore understood that the amount of W is great at the surface of thesubstrate. Also, the relationship between the depth from the surface ofthe substrate and the concentration of each component was evaluated. Theresult is shown in FIG. 3.

[0107] In FIG. 3, dotted line 201 indicates the actually measured valueof the concentration of Cu in the substrate. Dotted line 204 indicatesan average of the actually measured values of the Cu concentrationindicated by dotted line 201 for every 20 μm in depth. Solid line 202indicates the actually measured value of the W concentration in thesubstrate. Solid line 205 shows the average of the measured values of Windicated by solid line 202 for every 20 μm in depth. Chain dotted line203 indicates the concentration of C inside and outside the substrate.The C concentration outside the substrate is the concentration of C inthe fixture jig to secure the substrate.

[0108] In FIG. 3, the depth from the interface is plotted along theabscissa, and an arbitrary amount is plotted along the ordinates.Therefore, the accurate ratio of the concentration of Cu to W is notrepresented. The same applies for subsequent FIGS. 4 and 6.

[0109] It is appreciated from FIG. 3 that the concentration of W and Cuis substantially constant within the substrate.

[0110] According to step A of FIG. 15, the above substrate was immersedfor five minutes in a solution of mixed acid (mixture of hydrofluoricacid and nitric acid in the weight ratio of 1:1) diluted with purewater. The concentration of Cu and W within the substrate was measured.The result is shown in FIG. 4.

[0111] In FIG. 4, dotted line 211 shows the measured value of the Cuconcentration in the substrate. Dotted line 214 shows the average of themeasured values of Cu indicated by dotted line 211 for every 20 μm indepth. Solid line 212 shows the measured value of the W concentrationwithin the substrate. Solid line 215 shows the average of the measuredvalues of the W concentration within the substrate indicated by solidline 212 for every 20 μm in depth. Although chain dotted line 213 showsthe concentration of C outside and inside the substrate, the Cconcentration outside the substrate is the C concentration in thefixture jig that supports the substrate. It is appreciated from FIG. 4that the Cu at the surface of the substrate is removed by the mixed acidso that the Cu concentration (Cu content) becomes lower as a function ofapproaching the surface of the substrate.

[0112] Following the process of scratching the surface of the substratewith diamond abrasive grains, a substrate 26 was placed on substrateholder 27 in hot filament CVD apparatus 1 of FIG. 1. Current wasconducted from AC power supply 24 to tungsten filament 25 to set thetemperature of tungsten filament 25 to approximately 2050° C.

[0113] Then, mixture gas of methane and hydrogen with the methaneconcentration of 1 mole % was introduced through gas inlet 22 intoreactor 21. The pressure within reactor 21 was maintained at 70 Torr.According to step B of FIG. 15, a thin diamond film layer was grown onsubstrate 26 over the period of 40 hours. Thus, a thin diamond filmlayer 31 shown in FIG. 16 was obtained. The obtained thin diamond filmlayer 31 was 24 μm in thickness. The warp of substrate 26 was 3 μm.

[0114] The surface of thin diamond film layer 31 was polished to have amirror face. Then, an X-ray generated from a CuKα vessel was directedonto the surface of thin diamond film layer 31 to obtain an X-raydiffraction chart. The obtained X-ray diffraction chart is shown in FIG.5.

[0115] Referring to FIG. 5, the ratio I_(W) (110)I_(Cu) (200) of thepeak intensity (height) I_(W) (110) of the (110) plane of W to the peakintensity (height) I_(Cu) (200) of the (200) plane of Cu was 119. Theratio of the peak intensity (height) I_(W) (211) of the (211) plane of Wto the peak intensity (height) I_(Cu) (200) of the (200) plane of Cu was50. The relationship between the depth from the interface of substrate26 and thin diamond film layer 31 and the concentration of eachcomponent was evaluated. The result is shown in FIG. 6.

[0116] In FIG. 6, dotted line 221 shows the measured value of the Cuconcentration in the substrate. Dotted line 224 shows the average of themeasured values of the Cu concentration indicated by dotted line 221 forevery 20 μm in depth. Solid line 222 shows the measured value of the Wconcentration inside the substrate. Solid line 225 shows the average ofthe measured values of W indicated by solid line 222 for every 20 μm indepth. Chain dotted line 223 shows the concentration of C inside andoutside the substrate.

[0117] It is appreciated from FIG. 6 that the Cu concentration (content)becomes smaller as a function of coming closer to the surface of thesubstrate. The Cu content at the region located within 10 μm in depthfrom the surface of the substrate was not more than 50% of the entire Cucontent (11% by weight) of the substrate.

[0118] Thin diamond film layer 31 did not peel off substrate 26 evenwhen substrate 26 was cut to the size of 2 mm×1 mm×0.635 mm(lengthwise×breadthwise×thickness). Then, a laser diode was mounted onthe heatsink produced by applying metallization. The laser diodeexhibited stable oscillation. It is therefore appreciated that thisheatsink is sufficient for practical usage.

COMPARATIVE EXAMPLE 1

[0119] A substrate was prepared formed of a Cu—W sintered compact withthe Cu content of 11% by weight in the size of 13.5 mm×13.5 mm×0.635 mm(lengthwise×breadthwise×thickness). This substrate was immersed for 30seconds in a solution of nitric acid diluted with pure water. A processof scratching the surface of the substrate with diamond abrasive grainswas carried out. Then, substrate 26 was placed on substrate holder 27 inhot filament CVD apparatus 1.

[0120] The temperature of tungsten filament 25 was set to approximately2050° C. Mixture gas of methane and hydrogen with the methaneconcentration of 1 mole % was introduced into reactor 21 through gasinlet 22. The pressure in reactor 21 was 70 Torr. A thin diamond filmlayer was grown on the substrate over 40 hours under the aboveconditions. The thin diamond film layer was 23.5 μm in thickness and thewarp of the substrate was 3.4 μm.

[0121] The thin diamond film layer had its surface polished to have amirror face and then irradiated with an X-ray generated from a CuKαvessel to obtain an X-ray diffraction chart. The ratio of the peakintensity (height) of the (110) plane of W to the peak intensity of the(200) plane of Cu was obtained from the X diffraction chart. The peakintensity ratio was 65. This substrate was cut out to the size of 2 mm×1mm×0.635 mm (lengthwise×breadthwise×thickness). It was found that thethin diamond film layer peeled off the substrate.

EXAMPLE 2

[0122] A substrate 26 was prepared formed of a Cu—W sintered compactwith the Cu content of 15% by weight and 13.5 mm×13.5 mm×0.635 mm insize (lengthwise×breadthwise×thickness) as shown in FIG. 17. The surfaceof substrate 26 was roughened so that the surface roughness R_(Z) of thesubstrate was 1 μm. Here, R_(Z) refers to the ten point height ofirregularities defined by JIS (Japanese Industrial Standards).

[0123] On the surface subjected to the above roughening process, SiC 32as shown in FIG. 17 was deposited to 3 μm in thickness as anintermediate layer not including Cu. The surface of the intermediatelayer was scratched with diamond abrasive grains. Then, substrate 26 wasmounted on substrate holder 27 in hot filament CVD apparatus 1 ofFIG. 1. The temperature of tungsten filament 25 was set to approximately2100° C. Mixture gas of methane and hydrogen with the methaneconcentration of 1 mole % was introduced into reactor 21 through gasinlet 22. The pressure within reactor 21 was set to 70 Torr. A thindiamond film layer 31 as shown in FIG. 17 was grown on substrate 26 over40 hours under the above conditions. The obtained thin diamond filmlayer 31 had a thickness of 22 μm. The warp of substrate 26 was 2.5 μm.Thin diamond film layer 31 had its surface polished to have a mirrorface and then irradiated with an X-ray generated from a CuKα vessel toobtain an X-ray diffraction chart. From this X-ray diffraction chart,the ratio I_(W) (211)/I_(Cu) (200) of the peak intensity (height) I_(W)(211) by the (211) plane of W to the peak intensity (height) I_(Cu)(200) of the (200) plane of Cu was 47.

[0124] Then, the substrate was cut to the size of 2 mm×1 mm×0.635 mm(lengthwise×breadthwise×thickness). However, thin diamond film layer 31did not peel off substrate 26. Then, the plane of substrate 26 oppositeto the plane where thin diamond film layer 31 is formed was polished,and then subjected to metallization to produce a heatsink. A laser diodewas installed on the heatsink. The laser diode exhibited oscillation. Itis therefore appreciated that this heatsink is sufficient for practicalusage.

COMPARATIVE EXAMPLE 2

[0125] A substrate of 13.5 mm×13.5 mm×0.635 mm(lengthwise×breadthwise×thickness) in size was prepared, formed of aCu—W sintered compact with the Cu content of 15% by weight. The surfaceof this substrate was subjected to a roughening process so that thesurface roughness R_(Z) of the surface of the substrate was 5 μm.Following the roughening process, SiC of 12 μm in thickness wasdeposited as an intermediate layer not including Cu on the surface.

[0126] The surface of the intermediate layer was subjected to a processof scratching the surface with diamond abrasive grains. Substrate 26 wasplaced on substrate holder 27 in hot filament CVD apparatus 1 shown inFIG. 1. The temperature of tungsten filament 25 was set to approximately2100° C. Mixture gas of methane and hydrogen with the methaneconcentration of 1 mole % was introduced into reactor 21 through gasinlet 22. The pressure in reactor 21 was set to 70 Torr. A thin diamondfilm layer was grown on the substrate over 40 hours under the aboveconditions. The thickness of the thin diamond film layer was 23 μm. Thewarp of the substrate was 3.5 μm.

[0127] The thin diamond film layer had its surface polished to result ina mirror face, and then irradiated with an X-ray generated from a CuKαvessel to obtain an X-ray diffraction chart. According to this X-raydiffraction chart, the ratio I_(W) (211)/I_(Cu) (200) of the peakintensity (height) I_(W) (211) of the (211) plane of W to the peakintensity (height) I_(Cu) (200) of the (200) plane of Cu was 18. Thesubstrate was cut out to the size of 2 mm×1 mm×0.635 mm(lengthwise×breadthwise×thickness). The thin diamond film layer peeledoff the substrate.

EXAMPLE 3

[0128]FIG. 7 is a schematic diagram showing a microwave plasma CVDapparatus for diamond vapor synthesis employed in the present invention.Microwave plasma CVD apparatus 100 includes a substrate holder 101, amicrowave power source 104, a tuner 105, a wave guide 106, a reactor107, an outlet 108, an inlet 109, and a plunger 110.

[0129] Substrate holder 101 for supporting a substrate is providedwithin reactor 107. Reactor 107 includes inlet 109 from which materialgas is introduced and outlet 108 from which the material gas or gasgenerated by the reaction is output. Outlet 108 is connected to a vacuumpump. Microwave power source 104, an isolator (not shown) and a tuner105 form the microwave generation unit. Reactor 107 is formed of aquartz tube.

[0130] The microwave generated from the microwave generation unit isdirected towards plunger 110 via wave guide 106. Since reactor 10 7 isprovided in the passage of wave guide 106, plasma is generated withinreactor 107 as indicated by the circled dotted line 103. Plasma isgenerated at the area where reactor 107 crosses wave guide 106.Therefore, substrate holder 101 is provided in the proximity of thiscrossing position.

[0131] A substrate of 13.5 mm×13.5 mm×1 mm(lengthwise×breadthwise×thickness) was prepared, formed of a Cu—Wsintered compact with the Cu content of 11% by weight. This substratewas immersed for two minutes in a solution of mixed acid (mixture ofhydrofluoric acid and nitric acid in the weight ratio of 1:1) dilutedwith pure water. The surface of the substrate was subjected to theprocess of scratching the surface with diamond abrasive grains. Then,substrate 102 was placed on substrate holder 101 in microwave plasma CVDapparatus 100.

[0132] The temperature of substrate 102 was set to 850° C. Mixture gasof methane and hydrogen with the methane concentration of 3 mole % wasintroduced through inlet 109. The pressure within reactor 107 wasmaintained at 140 Torr. Plasma was generated in reactor 107. A thindiamond film layer was formed on substrate 102 over 20 hours under theabove conditions. The thin diamond film layer had a thickness of 22 μm.The warp of the substrate was 4 μm.

[0133] The thin diamond film layer had its surface polished to have amirror face, and then irradiated with an X-ray generated from a CuKαvessel to obtain an X-ray diffraction chart. The obtained X-raydiffraction is shown in FIG. 8.

[0134] According to the chart of FIG. 8, the ratio I_(W) (110)/I_(Cu)(200) of the peak intensity (height) I_(W) (110) of the (110) plane of Wto the peak intensity (height) I_(Cu) (200) of the (200) plane of Cu was140. The ratio I_(W) (211)/I_(Cu) (200) of the peak intensity (height)I_(W) (211) of the (211) plane of W to the peak intensity height) I_(Cu)(200) of the (200) plane of Cu was 50.

[0135] Even when a plurality of substrates were formed by cutting outthe substrate at the size of 2 mm×1 mm×1 mm(lengthwise×breadthwise×thickness), the thin diamond film layer did notpeel off the substrate. One of the plurality of substrates had the thindiamond film layer peeled off intentionally to observe an area range of3×4 μm² of the surface of the substrate with a 3D-SEM (three dimensionalscanning electron microscope) of the ERA 8000 type from ELIONIX. Theobserved result is shown in FIG. 9.

[0136] W particles were exposed at the surface of the substrate from theobserved result. The surface roughness R_(Z) of the W particle wasmeasured. The surface roughness R_(Z) was 0.09 μm. Another area of thesurface of the substrate was observed with a FESEM (field emissionscanning electron microscope). The observed result is shown in FIG. 10.

[0137] One of the plurality of substrates cut up had the surfaceopposite to the surface where the thin diamond film layer is formedpolished and subjected to metallization to produce a heatsink. A laserdiode was installed on this heatsink. The laser diode exhibited stableoscillation. It is therefore appreciated that this heatsink issufficient for practical usage.

EXAMPLE 4

[0138] A porous body of 10 mm×10 mm×0.3 mm(lengthwise×breadthwise×thickness) in size was prepared, formed of a Wsintered compact with the porosity of 27.5% by volume. Cu was permeatedinto the hole of the porous body. Accordingly, the entire Cu content ofthe substrate was set to 10% by weight, and the Cu content at the regionof 10 μm in depth from the surface where a thin diamond film layer is tobe formed was set to 3% by weight.

[0139] The surface of the substrate was subjected to a scratchingprocess using diamond abrasive grains. Substrate 102 was mounted onsubstrate holder 101 in microwave plasma CVD apparatus 100 shown in FIG.7. The temperature of substrate 102 was set to 850° C. Mixture gas ofmethane and hydrogen with the methane concentration of 3.5 mole % wasintroduced into reactor 107 through inlet 109. The pressure in reactor107 was set to 140 Torr. A thin diamond film layer was grown onsubstrate 102 over 20 hours. The thin diamond film layer had a thicknessof 25 μm. The warp of the substrate was 2.7 μm.

[0140] The surface of the thin diamond film layer was polished to have amirror face. The substrate was cut to the size of 2 mm×mm×0.3 mm(lengthwise×breadthwise×thickness). The thin diamond film layer did notpeel off the substrate. Then, the substrate was subjected tometallization to produce a heatsink. A laser diode was installed on thisheatsink. The laser diode exhibited stable oscillation. It is thereforeappreciated that this heatsink is sufficient for practical usage.

EXAMPLE 5

[0141] A substrate of 10 mm×10 mm×0.3 mm(lengthwise×breadthwise×thickness) was prepared, formed of a Cu—W—Mosintered compact with the Cu content of 15% by weight. This substratewas immersed for three minutes in a solution of nitric acid diluted withpure water. The surface of this substrate was subjected to a scratchingprocess with diamond abrasive grains. Then, substrate 206 was mounted onsubstrate holder 27 in hot filament CVD apparatus 1 shown in FIG. 1.

[0142] The temperature of tungsten filament 25 was set to approximately2100° C. Mixture gas of methane and hydrogen with the methaneconcentration of 2 mole % was introduced into reactor 21 through gasinlet 22. The pressure in reactor 21 was set to 70 Torr. A thin diamondfilm layer was grown on the substrate over 40 hours under the aboveconditions. The thin diamond film layer had a thickness of 22 μm. Thewarp in the substrate was 3 μm.

[0143] The thin diamond film layer had its surface polished to have amirror face, and then irradiated with an X-ray generated from a CuKαvessel to obtain an X-ray diffraction chart. According to this X-raydiffraction chart, the ratio I_(W) (110)/I_(Cu) (200) of the peakintensity (height) I_(w) (110) of the (110) plane of W to the peakintensity (height) I_(Cu) (200) of the (200) plane of Cu was 120.

[0144] This substrate was cut to the size of 2 mm×1 mm×0.3 mm(lengthwise×breadthwise×thickness). The thin diamond film layer did notpeel off the substrate. Then, the face of the substrate opposite to theface where the thin diamond film layer is formed was subjected tometallization to produce a heatsink. A laser diode was installed on thisheatsink. This laser diode exhibited oscillation. It is thereforeappreciated that this heatsink is sufficient for practical usage.

EXAMPLE 6

[0145] A substrate of 13.5 mm×13.5 mm×0.6 mm(lengthwise×breadthwise×thickness) was prepared formed of a Cu—Wsintered compact with the Cu content of 11% by weight. This substratewas immersed in aqua regia (a solution having concentrated nitric acidand concentrated hydrochloric acid mixed at the volume ratio of 1:3) forapproximately eight minutes. The surface of this substrate was subjectedto a scratching process with diamond abrasive grains. Then, substrate 26was mounted on substrate holder 27 in hot filament CVD apparatus of FIG.1.

[0146] The temperature of tungsten filament 25 was set to approximately2000° C. Mixture gas of methane and hydrogen with the methaneconcentration of 2 mole % was introduced into reactor 21 through gasinlet 22. The pressure within reactor 21 was maintained at 60 Torr. Athin diamond film layer was grown on substrate 26 over 45 hours underthe above conditions. The obtained thin diamond film layer had athickness of 25 μm. The warp of the substrate was 2 μm.

[0147] The thin diamond film layer had its surface polished to have amirror face, and then irradiated with an X-ray generated from a CuKαvessel to obtain an X-ray diffraction chart. According to the obtainedX-ray diffraction chart, the ratio I_(W) (211)/I_(Cu) (200) of the peakintensity (height) I_(W) (211) of the (211) plane of W to the peakintensity (height) I_(Cu) (200) of the (200) plane of Cu was 45.

[0148] The substrate was cut to the size of 2 mm×1 mm×0.6 mmlengthwise×breadthwise×thickness). The thin diamond film layer did notpeel off the substrate. Then, the substrate was subjected tometallization to produce a heatsink. A laser diode was installed on theheatsink. This laser diode exhibited stable oscillation. It is thereforeappreciated that this heatsink is sufficient for practical usage.

EXAMPLE 7

[0149] A substrate of 13.5 mm×13.5 mm×0.635 mm(lengthwise×breadthwise×thickness) was prepared formed of a Cu—Wsintered compact with the Cu content of 11% by weight. Si of 5 μm inthickness was deposited as an intermediate layer not including Cu on thesurface of this substrate. This intermediate layer was subjected to ascratching process with diamond abrasive grains. Substrate 102 wasplaced on substrate holder 101 in microwave plasma CVD apparatus 100shown in FIG. 7.

[0150] The temperature of substrate 102 was set to approximately 900° C.Mixture gas of methane and hydrogen with the methane concentration of2.5 mole % was introduced into reactor 107 through gas inlet 109. Thepressure within reactor 107 was maintained at 100 Torr. A thin diamondfilm layer was grown on substrate 102 over 30 hours. The thickness ofthe thin diamond film layer was 23 μm. The warp of the substrate was 4μm.

[0151] The thin diamond film layer had its surface polished to have amirror face, and then irradiated with an X-ray generated from a CuKαvessel to obtain an X-ray diffraction chart. According to the X-raydiffraction chart, the ratio I_(W) (110)/I_(Cu) (200) of the peakintensity (height) I_(W) (110) of the (110) plane of W to the peakintensity (height) I_(Cu) (200) of the (200) plane of Cu was 130. Asubstrate of 2 mm×1 mm×0.635 mm (lengthwise×breadthwise×thickness) wascut. The thin diamond film layer did not peel off the substrate. Then,the substrate was subjected to metallization to produce a heatsink. Alaser diode was installed on the heatsink. This laser diode exhibitedstable oscillation. It is therefore appreciated that this heatsink issufficient for practical usage.

EXAMPLE 8

[0152] A substrate of 13.5 mm×13.5 mm×0.6 mm(lengthwise×breadthwise×thickness) formed of a Cu—W sintered compactwith the Cu content of 15% by weight was prepared. This substrate wasimmersed for one minute in a solution of mixed acid (mixture ofhydrofluoric acid and nitric acid in the weight ratio of 1:1) dilutedwith pure water. The surface of this substrate was subjected to ascratching process with diamond abrasive grains. Then, substrate 26 wasplaced on substrate holder 27 in hot filament CVD apparatus 1 of FIG. 1.

[0153] The temperature of tungsten filament 25 was set to approximately2100° C. Mixture gas of methane and hydrogen with the methaneconcentration of 1 mole % was introduced into reactor 21 through gasinlet 22. The pressure in reactor 21 was set to 70 Torr. A thin diamondfilm layer was grown on substrate 26 over 44 hours under the aboveconditions. The thin diamond film layer had a thickness of 21 μm. Thewarp of the substrate was 3 μm.

[0154] The thin diamond film layer had its surface polished to have amirror face, and then irradiated with an X-ray generated from a CuKαvessel to obtain an X-ray diffraction chart. According to the X-raydiffraction chart, the ratio I_(W) (110)/I_(Cu) (200) of the peakintensity (height) I_(W) (110) of the (110) plane of W to the peakintensity (height) I_(Cu) (200) of the (200) plane of Cu was 121. Aplurality of WC peaks appeared in this X-ray diffraction chart.

[0155] The substrate was cut to the size of 2 mm×1 mm×0.6 mm(lengthwise×breadthwise×thickness). However, the thin diamond film layerdid not peel off the substrate. Then, the substrate was subjected tometallization to produce a heatsink. A laser diode was installed on theheatsink. This laser diode exhibited stable oscillation. It is thereforeappreciated that this heatsink is sufficient for practical usage.

EXAMPLE 9

[0156] In Example 3, the surface roughness R_(Z) of the W particle onthe surface of the substrate was set to 0.09 μm by immersing thesubstrate in mixed acid. In Example 9, a method of roughening thesurface of the W particle by a method other than that of the mixed acidprocess was studied. As a result, it was found that the surface of the Wparticle could be roughened by the following five methods.

[0157] 1: Shot blasting of spraying fine particles such as of metal onthe substrate

[0158] 2: Argon sputtering of converting argon gas into plasma, andapplying bias on the substrate to have argon atoms collide with thesubstrate

[0159] 3: Alkali process of immersing the substrate in alkali

[0160] 4: Fluorine plasma process of exposing the substrate to fluorineplasma

[0161] 5: Electron beam irradiation of irradiating the substrate with anelectron beam

EXAMPLE 10

[0162] A substrate of 13.5 mm×13.5 mm×0.6 mm(lengthwise×breadthwise×thickness) in size formed of a Cu—W sinteredcompact with the Cu content of 11% by weight was prepared. Thissubstrate was immersed for 30 minutes in a solution of nitric aciddiluted with pure water. Then, the surface of the substrate wasirradiated with an X-ray to obtain an X-ray diffraction chart. No Cupeak was observed in this X-ray diffraction chart.

[0163] The surface of the substrate was subjected to a scratchingprocess with diamond abrasive grains. Then, substrate 26 was placed onsubstrate holder 27 in hot filament CVD apparatus 1 shown in FIG. 1.Then, current was conducted from AC power supply 24 to tungsten filament25, whereby the temperature of tungsten filament 25 was set toapproximately 2050° C.

[0164] Then, mixture gas of methane and hydrogen with the methaneconcentration of 1 mole % was introduced into reactor 21 from gas inlet22. A thin diamond film layer was grown on substrate 26 over 40 hoursunder the conditions that the pressure in reactor 21 was maintained at70 Torr. The thin diamond film layer had a thickness of 24 μm. The warpof the substrate was 3 μm.

[0165] The thin diamond film layer was polished to have a mirror plane.Then the substrate was cut to the size of 2 mm×1 mm×0.6 mm(lengthwise×breadthwise×thickness). The thin diamond film layer did notpeel off the substrate.

[0166] Then, the substrate was subjected to metallization to produce aheatsink. A laser diode was installed on the heatsink. This laser diodeexhibited oscillation. It is therefore appreciated that this heatsink issufficient for practical usage.

EXAMPLE 11

[0167] A substrate of 10 mm×10 mm×0.6 mm(lengthwise×breadthwise×thickness) formed of a Cu—W sintered compactwith the Cu content of 15% by weight was prepared. This substrate wasimmersed for three minutes in a solution of mixed acid (hydrofluoricacid and nitric acid mixed at the volume ratio of 1:1) with pure water.The surface roughness R_(Z) of the substrate was 4.5 μm. The surface ofthe substrate subjected to acid treatment was observed with a 3D-SEM(three dimensional scanning electron microscope) of the ERA 8000 typefrom ELIONIX. The observed result is shown in FIG. 11.

[0168] According to FIG. 11, W particles are exposed at the surface ofthe substrate subjected to acid treatment. The surface roughness of theexposed W particle was 0.08 μm.

[0169] The surface of this substrate was observed by a FESEM (fieldemission scanning electron microscope). The observed result is shown inFIG. 12.

[0170] The surface of the substrate subjected to acid treatment wasscratched with diamond grains. Then, substrate 26 was placed onsubstrate holder 27 in hot filament CVD apparatus 1 of FIG. 1.

[0171] The temperature of tungsten filament 25 was set to approximately2100° C. Mixture gas of methane and hydrogen with the methaneconcentration of 1 mole % was introduced into reactor 21 through gasinlet 22. The pressure in reactor 21 was maintained at 70 Torr. A thindiamond film layer was grown on substrate 26 over 40 hours under theabove conditions. The thin diamond film layer had a thickness of 22 μm.The warp of the substrate was 2.5 μm.

[0172] Then, the surface of the thin diamond film layer was polished tohave a mirror plane. The substrate was cut to the size of 2 mm×1 mm×0.6mm (lengthwise×breadthwise×thickness). However, the thin diamond filmlayer did not peel off the substrate.

[0173] Then, the surface of the substrate opposite to the surface wherethe thin diamond film layer is formed was polished and then subjected tometallization to produce a heatsink. A laser diode was placed on theheatsink. This laser diode exhibited stable oscillation. It is thereforeappreciated that this heatsink is sufficient for practical usage.

EXAMPLE 12

[0174] A substrate of 13.5 mm×13.5 mm×0.6 mm(lengthwise×breadthwise×thickness) formed of a Cu—W compact with the Cucontent of 15% by weight was prepared. This substrate was immersed in asolution of hydrogen peroxide for one minute, then in nitric acid forfive minutes. The surface roughness R_(Z) of the substrate was 4.5 μm.The surface of the substrate was subjected to a scratching process bydiamond grains. Substrate 26 was placed on substrate holder 27 in hotfilament CVD apparatus 1 shown in FIG. 1.

[0175] The temperature of tungsten filament 25 was set to approximately2100° C. Mixture gas of methane and hydrogen with the methaneconcentration of 1 mole % was introduced into reactor 21 through gasinlet 22. The pressure in reactor 21 was set to 70 Torr. A thin diamondfilm layer was grown on substrate 26 over 40 hours under the aboveconditions. The thin diamond film layer had a thickness of 21 μm. Thewarp of the substrate was 2.5 μm.

[0176] The surface of the thin diamond film layer was polished to have amirror plane. The substrate was cut to the size of 2 mm×1 mm×0.6 mm(lengthwise×breadthwise×thickness). The thin diamond film layer did notpeel off the substrate. Then, the surface of the substrate opposite tothe surface where the thin diamond film layer is formed was polished andsubjected to metallization to produce a heatsink. A laser diode wasinstalled on this heatsink. This laser diode exhibited oscillation. Itis therefore appreciated that this heatsink is sufficient for practicalusage.

EXAMPLE 13

[0177] A substrate of 15 mm×15 mm×1 mm(lengthwise×breadthwise×thickness) formed of a Cu—Mo sintered compactwith the Cu content of 10% by weight was prepared. This substrate wasimmersed in sulfuric acid for thirty minutes. Accordingly, the porosityat the region of the substrate within 30 μm in depth from the surface ofthe substrate was 25% by volume, and the Cu content at the region of thesubstrate within 30 μm in depth from the surface was 2% by weight. It isto be noted that the entire Cu content of the substrate was still 10% byweight.

[0178] The surface of the substrate was subjected to a scratchingprocess with diamond grains. Then, substrate 102 was placed on substrateholder 101 in microwave plasma CVD apparatus 100 shown in FIG. 4. Thetemperature of substrate 102 was set to approximately 850° C. Mixturegas of methane and hydrogen with the methane concentration of 3 mole %was introduced into reactor 107 through inlet 109. The pressure inreactor 107 was maintained at 140 Torr.

[0179] A thin diamond film layer was grown on substrate 102 over 20hours under the above conditions. The thin diamond film layer had athickness of 22 μm. The warp of the substrate was 4 μm.

[0180] The surface of the thin diamond film layer was polished to have amirror plane. Then, the substrate was cut to the size of 2 mm×1 mm×1 mm(lengthwise×breadthwise×thickness). The thin diamond film layer did notpeel off the substrate. Then, the surface of the substrate opposite tothe surface where the thin diamond film layer is formed was polished andsubjected to metallization to produce a heatsink. A laser diode wasinstalled on this heatsink. This laser diode exhibited oscillation. Itis therefore appreciated that this heatsink is sufficient for practicalusage.

EXAMPLE 14

[0181] A substrate of 10 mm×10 mm×0.3 mm(lengthwise×breadthwise×thickness) in size formed of a Cu—W sinteredcompact with the Cu content of 15% by weight was prepared. Thissubstrate was immersed in hydrochloric acid for forty minutes. Thesurface roughness R_(Z) of the substrate resulted in 3.6 μm.

[0182] The surface of the substrate was subjected to a scratchingprocess with diamond grains. Substrate 102 was placed on substrateholder 101 in the microwave plasma CVD apparatus shown in FIG. 7. Thetemperature of substrate 102 was set to approximately 850° C. Mixturegas of methane and hydrogen with the methane concentration of 3.5 mole %was introduced into reactor 107 through inlet 109. The pressure inreactor 107 was maintained at 140 Torr. A thin diamond film layer wasgrown on substrate 102 over 20 hours under the above conditions. Thethickness of the thin diamond film layer was 25 μm. The warp of thesubstrate was 2.7 μm.

[0183] The surface of the thin diamond film layer was polished to have amirror plane. Then, the substrate was cut to the size of 2 mm×1 mm×0.3mm (lengthwise×breadthwise×thickness). The thin diamond film layer didnot peel off the substrate. Then, the surface of the substrate oppositeto the surface where the thin diamond film layer is formed was polishedand subjected to metallization to form a heatsink. A laser diode wasinstalled on this heatsink. This laser diode exhibited oscillation. Itis therefore appreciated that this heatsink is sufficient for practicalusage.

EXAMPLE 15

[0184] A substrate of 10 mm×10 mm×0.6 mm(lengthwise×breadthwise×thickness) in size formed of a Cu—W sinteredcompact with the Cu content of 11% by weight was prepared. Thissubstrate was immersed for one minute in a solution of mixed acid(mixture of hydrofluoric acid and nitric acid in the weight ratio of1:1) diluted with pure water. Then, the substrate was immersed for fiveminute in nitric acid. The substrate was irradiated with an X-raygenerated from a CuKα vessel to obtain an X-ray diffraction chart. ThisX-ray diffraction chart is shown in FIG. 13.

[0185] It is appreciated from FIG. 13 that no Cu peak appears in theX-ray diffraction chart of the surface of the substrate subjected toacid treatment.

[0186] Then, the surface of this substrate is subjected to a scratchingprocess using diamond grains. Then, substrate 26 was placed on substrateholder 27 in hot filament-CVD apparatus 1 shown in FIG. 1. Then,temperature of tungsten filament 25 was set to approximately 2100° C.Mixture gas of methane and hydrogen with the methane concentration of 1mole % was introduced into reactor 21 through gas inlet 22. The pressureof reactor 21 was set to 70 Torr. A thin diamond film layer was formedon substrate 26 over 40 hours under the above conditions. The thicknessof the thin diamond film layer was 20 μm. The warp of the substrate was2.5 μm.

[0187] The surface of the thin diamond film layer was polished to have amirror plane. Then the substrate was cut to the size of 2 mm×1 mm×0.6 mm(lengthwise×breadthwise×thickness). The thin diamond film layer did notpeel off the substrate. Then, the surface of the substrate opposite tothe surface where the thin diamond film layer is formed was polished sothat the thickness of the substrate was 0.3 mm. This substrate wassubjected to metallization to produce a heatsink. A laser diode wasinstalled on the heatsink. This laser diode exhibited oscillation. It istherefore appreciated that this heatsink is sufficient for practicalusage.

COMPARATIVE EXAMPLE 3

[0188] In Comparative Example 3, the condition of the acid treatment ofExample 15 was altered. In contrast to Example 15 in which the substratewas immersed for one minute in mixed acid diluted with pure water andthen immersed for five minutes in nitric acid, Comparative Example 3 hadthe substrate immersed for ten seconds in mixed acid diluted with purewater, but was not immersed in nitric acid thereafter. A thin diamondfilm layer was grown on the substrate subjected to acid treatment underthe conditions similar to those of Example 15. This thin diamond filmlayer was irradiated with an X-ray generated from a CuKα vessel toobtain an X-ray diffraction chart. This X-ray diffraction chart is shownin FIG. 14.

[0189] It is appreciated that there is a Cu peak in the X-raydiffraction chart of FIG. 14. The surface of the diamond was polished tohave a mirror plane. Then the substrate was cut to the size of 2 mm×1mm×0.6 mm (lengthwise×breadthwise×thickness). A portion of the thindiamond film layer peeled off the substrate.

EXAMPLE 16

[0190] A porous body 310 as shown in FIG. 18 in the size of 100 mm×80mm×1 mm (lengthwise×breadthwise×thickness), formed of a tungsten metalsintered compact with the porosity of 21% by volume was prepared. Thesurface of porous-body 310 including a hole 311 was subjected to ascratching process with diamond grains. Then, porous body 310 wasmounted on substrate holder 27. Then, current was conducted from ACpower supply 24 to tungsten filament 25, whereby the temperature oftungsten filament 25 was set to approximately 2050° C.

[0191] Then, mixture gas of hydrogen and methane with the methaneconcentration of 1 mole % was introduced into reactor 21 through gasinlet 22. A thin diamond film layer 320 as shown in FIG. 19 was grown onporous body 310 over 40 hours under the condition that the pressure inreactor 21 was maintained at 70 Torr. The thickness of thin diamond filmlayer 320 was 24 μm. Then, a Cu plate was mounted on a heater forheating. Porous body 310 on which thin diamond film layer 320 is formedwas mounted on this Cu plate.

[0192] The Cu was heated using the heater up to the temperature of 1100°C. to be melted, whereby the Cu permeated into hole 311 of porous body310 over the period of ten hours to form a substrate. The substrateincludes the porous body and Cu. Then, the substrate was cut to the sizeof 10 mm×10 mm×1 mm (lengthwise×breadthwise×thickness). The thin diamondfilm layer was polished to have a mirror plane. The substrate was cut tothe size of 2 mm×1 mm×1 mm lengthwise×breadthwise×thickness) to obtain aheatsink. The thin diamond film layer did not peel off the porous body.The thin diamond film layer and the substrate were subjected tometallization to obtain a heatsink shown in FIG. 20.

[0193] Referring to FIG. 20, a heatsink 300 is formed of porous body 310and thin diamond film layer 320. Porous body 310 is formed of a tungstenmetal sintered compact which is a sintered version of fine tungstenmetal powder. A plurality of holes 311 are present in porous body 310.All these holes 311 communicate with each other. Holes 311 are filledwith Cu 312. At the surface layer 310 a of the porous body, diamond filmlayer 320 permeates into hole 311.

[0194] A polycrystalline diamond film layer 320 is formed at the surfaceof porous body 310.

[0195] A laser diode was installed on this heatsink 300. This laserdiode exhibited oscillation. It is therefore appreciated that thisheatsink is sufficient for actual practice.

EXAMPLE 17

[0196] A porous body of 100 mm×80 mm×2 mm(lengthwise×breadthwise×thickness)in size, formed of a porous tungstenmetal sintered compact with the porosity of 28% by volume was prepared.The surface of porous body was subjected to a scratching process withdiamond grains. Then, the porous body was mounted on substrate holder 27shown in FIG. 1. The temperature of tungsten filament 25 was set toapproximately 2100° C. Mixture gas of hydrogen and methane with themethane concentration of 1 mole % was introduced into reactor 21 throughgas inlet 22. The pressure in reactor 21 was set to 70 Torr. A thindiamond film layer was formed on the porous body over 40 hours under theabove conditions. The thickness of the thin diamond film layer was 22μm.

[0197] Then, the porous body was placed on substrate holder 27 with thethin diamond film layer downwards. A Cu plate was placed on the porousbody. A heater was placed on this Cu plate to heat the Cu plate up tothe temperature of 1100° C. to melt the Cu. Accordingly, the Cupermeated into the porous body to form a substrate. The substrateincludes the porous body and Cu. Then, the substrate was cut to the sizeof 10 mm×10 mm×2 mm (lengthwise×breadthwise×thickness). The thin diamondfilm layer had its surface polished to have a mirror plane. Then, thesubstrate was cut to the size of 2 mm×1 mm×1 mm(lengthwise×breadthwise×thickness). However, the thin diamond film layerdid not peel off the substrate.

[0198] The plane of the substrate opposite to the plane where the thindiamond film layer is formed was polished and subjected to metallizationto produce a heatsink. A laser diode was installed on this heatsink.This laser diode exhibited oscillation. It is therefore appreciated thatthis heatsink is applicable to practice usage.

EXAMPLE 18

[0199] A porous body of 100 mm×80 mm×2 mm in size(lengthwise×breadthwise×thickness) formed of a porous tungsten metalsintered compact with the porosity of 35% by volume was prepared. Thesurface of the porous body was subjected to a scratching process withdiamond grains. This porous body was placed on substrate holder 27 shownin FIG. 1. The temperature of tungsten filament 25 was set toapproximately 2100° C. Mixture gas of hydrogen and methane with themethane concentration of 1 mole % was introduced into reactor 21 throughgas inlet 22. The pressure of reactor 21 was set to 70 Torr. A thindiamond film layer was grown on porous body over 40 hours under theabove conditions. The thickness of the thin diamond film layer was 22μm.

[0200] Then, a crucible was prepared as a container. Cu was introducedinto this crucible. The Cu was heated up to the temperature of 1100° C.to be melted. The porous body was dipped in the crucible to have the Cupermeate into the hole of the porous body to form a substrate. Thesubstrate includes the porous body and Cu. Then, the substrate was cutto the size of 10 mm×10 mm×2 mm lengthwise×breadthwise×thickness). Thesurface of the thin diamond film layer was polished to have a mirrorplane. Then, the substrate was further cut to the size of 2 mm×1 mm×1 mm(lengthwise×breadthwise×thickness). The plane of the substrate oppositeto the plane where the thin diamond film layer is formed was polishedand subjected to metallization to produce a heatsink. A laser diode wasinstalled on this heatsink. This laser diode exhibited oscillation. Itis therefore appreciated that this heatsink is applicable to practiceusage.

EXAMPLE 19

[0201] A radiator base 81 of 20 mm×20 mm×0.4 mm(lengthwise×breadthwise×thickness) formed of Si, AlN, CuW alloy or SiCas shown in FIG. 21 was prepared. One surface thereof was subjected to ascratching process using diamond powder. Then, diamond was grownentirely on that surface by hot filament CVD. The conditions for growthis set forth in the following. Material gas 1% by weightmethane-hydrogen Flow rate 600 sccm Pressure 80 Torr SubstrateTemperature 710° C. Filament Tungsten Filament Temperature 2150° C.

[0202] At one surface of each radiator base 81, a vapor synthesisdiamond layer 82 of high adherence was obtained. Each vapor synthesisdiamond layer 82 was polished to have respective thickness of 20 μm, 50μm, and 100 μm. The thermal conductivity of each obtained vaporsynthesis diamond layer 82 was measured by the laser flash method. Eachvapor synthesis diamond layer 82 had the thermal conductivity of 1310W/m·K.

[0203] Following the polishing process, each vapor synthesis diamondlayer 82 was cut by laser and had its surface subjected tometallization. All the metallized layers were Au3 μm/Pt0.05 μm/Ti0.1 μm.The opposite surface of radiator base 81 was metallized by depositingAu, and bound to a base metal member 83 formed of a CuW alloy with abrazing material. An MMIC formed of GaAs which is a semiconductorelement 84 is bound using a brazing material to the metallized layer onvapor synthesis diamond layer 82 with the heat generating region above.

[0204] As a comparative example, a diamond free-standing plate (20 mm×20mm×0.4 mm: lengthwise×breadthwise×thickness) and a BeO substrate (20mm×20 mm×0.4 mm: lengthwise×breadthwise×thickness) were prepared insteadof the heatsink formed of a layered member of the above vapor synthesisdiamond layer 82 and radiator base 81. In a similar manner, the basemetal member and the semiconductor element (MMIC) were bound.

[0205] Sample 13 using the diamond free-standing plate was cracked whenthe semiconductor element of GaAs was mounted. In contrast, respectivesamples 1-12 of the present invention exhibited no breakage of thesemiconductor element at the time of mounting by brazing. This isbecause the thermal expansion coefficient of samples 1-12 of the presentinvention is similar to that of GaAs. The mounted semiconductor element,i.e., MMIC, operated stably. Samples 1-12 and 14 that did not exhibitbreakage of the semiconductor element have low thermal resistance, asindicating in the following Table 1. Particularly, the thermalresistance of samples 1-12 of the present invention was lower than thatof sample 14 using BeO. TABLE 1 Diamond Thermal Thickness ResistanceSample Heatsink Structure (μm) (° C./W) 1 Diamond/Si 20 3.71 2Diamond/Si 50 3.51 3 Diamond/Si 100 3.27 4 Diamond/AIN 20 3.59 5Diamond/AIN 50 3.38 6 Diamond/AIN 100 3.10 7 Diamond/CuW 20 3.52 8Diamond/CuW 50 3.30 9 Diamond/CuW 100 2.99 10  Diamond/SiC 20 3.85 11 Diamond/SiC 50 3.65 12  Diamond/SiC 100 3.34 13* Diamond Free-standingplate 400 — 14* BeO Substrate — 4.35

EXAMPLE 20

[0206] In the manner similar to that of the above Example 19, a thindiamond film layer was grown by vapor synthesis on one surface of aradiator base 91 of 14 mm×14 mm×0.4 mm(lengthwise×breadthwise×thickness) in size as shown in FIG. 22. Thisthin diamond film layer was polished. A heatsink was produced havingvapor synthesis diamond layer 92 of 40 μm in thickness on radiator base91.

[0207] A polyimide-Cu multilayer interconnection layer (three layers) 95was provided on vapor synthesis diamond layer 92 of the heatsink. Then,excimer laser was focused on a predetermined position to carry out viahole processing to form an interlayer interconnection by the throughhole. Then, similar to Example 19, the radiator base 91 side of theheatsink was attached by brazing to metal member 93 based on CuW. MMICsemiconductor element 94 was bound by brazing at the element mountingarea of vapor synthesis diamond layer 92. Then, connection betweensemiconductor element 94 and multilayer interconnection layer 95 waseffected.

[0208] MMIC semiconductor element 94 mounted on this heatsink did notexhibit breakage during the stage of mounting. Semiconductor element 94operated stably for a long period of time. Superior heat dissipation wasexhibited.

[0209] The following issues are identified from the above Examples 19and 20. In order to prevent the semiconductor element from cracking, thesemiconductor module of the present invention includes a radiator baseof 200-700 μm in thickness with the thermal conductivity of at least 100W/m·K, a vapor synthesis diamond layer of 10-200 μm in thicknessprovided on the radiator base, and a high power semiconductor elementmounted on the vapor synthesis diamond layer.

[0210] In the semiconductor module of the present invention, theradiator base is formed of at least one type selected from the groupconsisting of Si, SiC, AlN, CuW alloy, CuMo alloy, and CuMoW alloy. Thebase may have the Cu concentration become lower as a function ofapproaching the surface, as used in Examples 1-18, or may have Cupermeated into the hole of a W porous body. The vapor synthesis diamondlayer has a thermal conductivity that is preferably at least 100 W/m·K.

[0211] As a specific structure related to mounting a semiconductorelement for the semiconductor module of the present invention, at leasta portion of the semiconductor element mounting surface of the vaporsynthesis diamond layer has a metallized layer, wherein the metallizedlayer is formed of at least one type selected from the group of Au, Mo,Ni, Pt, Pd, Ti, Cu, Al, and has a high power semiconductor element boundon the metallized layer by a brazing material.

[0212] In the semiconductor module of the present invention, the mountedhigh power semiconductor element has gallium, arsenide as the maincomponent. The semiconductor module of the present invention isparticularly suitable for mounting a high power transistor and a MMIC.The high power semiconductor element preferably has its plane oppositeto the heat generating region bound to the vapor synthesis diamond layerside.

[0213] In the semiconductor module of the present invention, thepackaging density of the semiconductor element can be further improvedby forming a multilayer interconnection layer including an insulationlayer having a dielectric constant of not more than 5 and a metalinterconnection layer at the plane side of the vapor synthesis diamondlayer where the high power semiconductor element is mounted.

[0214] In the semiconductor module of the present invention, a thinvapor synthesis diamond layer of 10-200 μm in thickness is arranged atthe plane side where the semiconductor element of the radiator baseserving as a heatsink or a radiator base is mounted. By arranging such athin vapor synthesis diamond layer on the radiator base, the heatgenerated from the semiconductor element mounted on this vapor synthesisdiamond layer can be dissipated efficiently. Also, the semiconductorelement can be prevented from being cracked at the time of mounting.

[0215] The heat generated by the semiconductor element is first diffusedlaterally within the vapor synthesis diamond layer of high thermalconductivity to be further diffused towards the radiator base from theentire surface of the vapor synthesis diamond layer. Therefore, heatdissipation of high efficiency can be applied to the module even if thevapor synthesis diamond layer is thin. The thermal conductivity of thevapor synthesis diamond layer is preferably at least 100 W/m·K in orderto achieve favorable heat dissipation in the horizontal direction in thevapor synthesis diamond layer.

[0216] Although the heat conductivity of the radiator base may besmaller than the heat conductivity of diamond, the heat dissipationeffect of the vapor synthesis diamond layer will be lost if the thermalconductivity is too low. Therefore, the thermal conductivity of theradiator base must be at least 100 W/m·K. As a radiator base with suchthermal conductivity, Si, SiC, AlN, Cu, CuW alloy, CuMo alloy, CuMoWalloy and the like is preferably used.

[0217] In general, diamond is degraded in mechanical strength as itbecomes thinner, and has a low thermal expansion coefficient. There wasa problem that the semiconductor element is cracked due to difference inthermal expansion when a semiconductor element formed mainly of galliumarsenide (GaAs) such as a MMIC is connected by brazing.

[0218] However, the present invention provides a thin coat of diamondthat is less than 200 μm in thickness by vapor synthesis on a radiatorbase that has a thermal expansion coefficient higher than that ofdiamond. Accordingly, the obtained vapor synthesis diamond layer has athermal expansion coefficient approaching that of the radiator basewithout having the mechanical strength degraded. Therefore, cracking ofthe semiconductor element can be suppressed even when a semiconductorelement of GaAs is brazed on the vapor synthesis diamond layer.

[0219] When the thickness is at least 10 μm, the vapor synthesis diamondlayer has sufficient mechanical strength and exhibits sufficient heatdissipation. Preferably, the thickness of the vapor synthesis diamondlayer is at least 20 μm. However, the cost will increase as the vaporsynthesis diamond layer becomes thicker. If the vapor synthesis diamondlayer becomes thicker than 200 μm, the influence of the underlyingradiator base will be lost, to degrade the thermal expansioncoefficient. As a result, the possibility of breakage in thesemiconductor element when mounted becomes higher. If the thickness ofthe radiator base is less than 200 μm, the mechanical strength willbecome too weak. If the thickness of the radiator base exceeds 700 μm,the entire heat dissipation of the module will be degraded. Therefore,the thickness of the radiator base is preferably in the range of 200-700μm, more preferably in the range of 250-500 μm.

[0220] Natural diamond or high pressure synthetic diamond can be usedfor the diamond layer provided on the radiator base. However, it isdifficult to bond the diamond layer and the base together. Degradationin heat dissipation Will occur. There was a problem that a diamond layerof a large area cannot be produced. According to the vapor synthesismethod, a thin diamond film layer can be directly grown on the radiatorbase. According to the present invention, a vapor synthesis diamondlayer with the required heat conductance and thickness can be obtainedeasily. Furthermore, the cost can be reduced considerably in comparisonto the case using natural diamond or high pressure synthetic diamond.

[0221] The layered member of the vapor synthesis diamond layer and theradiator base is used as the heatsink or the radiator substrate. A highpower semiconductor element such as a high power transistor or a MMIC ismounted on the vapor synthesis diamond layer. These high powersemiconductor elements have GaAs as the main component. Thesemiconductor element generally has a heat generation region at onesurface side. The semiconductor element can be mounted with the surfaceopposite to the side of this heat generation region facing the vaporsynthesis diamond layer mounted.

[0222] In order to mount a semiconductor element, a metallized layer isformed of at least one type selected from the group consisting of Au,Mo, Ni, Pt, Pd, Ti, Cu, Al, and the like for the semiconductor elementmounting plane of the vapor synthesis diamond layer. A high powersemiconductor element such as a high power transistor or a MMIC is fixedon the metallized layer by a brazing material such as AuSn, AuGe, andAuSi. The total thickness of these metallized layer and brazing layer ispreferably in the range of 0.1-50 μm.

[0223] The packaging density of the semiconductor element can further beimproved by interconnection with the semiconductor element according toa multilayer interconnection layer of an insulation layer and a metalinterconnection layer formed at the plane of the vapor synthesis diamondlayer where the semiconductor element is mounted. In this case, theinsulation layer preferably has a dielectric constant of not more than 5since an insulation of lower dielectric constant can have the noise bypropagation or the loss reduced.

[0224] The layered member of the above vapor synthesis diamond layer andradiator base is connected to the general base metal such as CuW at theradiator base side to be used as a heatsink or a radiator substrate. Inthis case, a metallized layer formed of at least one type selected fromthe group consisting of Au, Mo, Ni, Pt, Pd, Ti, Cu, Al, and the like isprovided at the surface side of the radiator base where the base metalis connected. The metallized layer is bound using a brazing materialsuch as AuSn and AuSi.

[0225] According to the present invention, a heatsink can be providedimproved in adherence between the thin diamond film layer and thesubstrate, with the warping reduced.

[0226] The present invention is advantageous in that the fabrication iscarried out without using toxic BeO. Since a thin vapor synthesisdiamond layer is used, the cost can be suppressed. A semiconductormodule superior in heat dissipation can be provided. The semiconductormodule has difference in thermal expansion with the semiconductorelement reduced by the radiator base and the vapor synthesis diamondlayer formed thereon to alleviate thermal stress in the element.Therefore, cracking in the semiconductor element can be suppressedduring the element mounting stage.

[0227] Although the present invention has been described and illustratedin detail, it is clearly understood that the same is by way ofillustration and example only and is not to be taken by way oflimitation, the spirit and scope of the present invention being limitedonly by the terms of the appended claims.

What is claimed is:
 1. A heatsink fabrication method comprising thesteps: a) immersing a surface of a substrate including Cu and a metalhaving a low thermal expansion coefficient in an acid to reduce a Cucontent of a region of a surface layer of said substrate, and to exposeand roughen an exposed surface of said metal at said region; and b)forming a thin diamond film layer by vapor synthesis on said region ofsaid surface layer.
 2. The heatsink fabrication method according toclaim 1, wherein said acid is an acid solution selected from the groupconsisting of hydrochloric acid, nitric acid, sulfuric acid,hydrofluoric acid, hydrogen peroxide, chromic acid, or a solution of amixture thereof.
 3. The heatsink fabrication method according to claim1, wherein said step a) includes a first acid treatment step ofimmersing said surface of said substrate in a first acid, and asubsequent second acid treatment step of immersing said surface of saidsubstrate in a second acid different from said first acid.
 4. Theheatsink fabrication method according to claim 1, wherein said substratecomprises a sintered compact selected from the group consisting of aCu—W sintered compact and a Cu—W—Mo sintered compact.
 5. The heatsinkfabrication method according to claim 1, wherein said substrate includesW particles that are exposed at said surface of said substrate that hasbeen immersed in acid, and wherein said W particles have a surfaceroughness R_(Z) of at least 0.05 μm.
 6. The heatsink fabrication methodaccording to claim 1, wherein said step a) to reduce said Cu content iscarried out until a Cu peak is not detected in an X-ray diffractionchart obtained by irradiating said surface of said substrate with anX-ray.
 7. The heatsink fabrication method according to claim 1, whereinsaid step a) is carried out until a region within 30 μm in depth fromsaid surface of said substrate has a porosity of at least 5% by volumeand not more than 70% by volume, and has a Cu content that is not morethan 50% of an entire Cu content of said substrate.
 8. The heatsinkfabrication method according to claim 1, further comprising a step ofscratching said surface of said substrate using diamond, prior to saidstep b).
 9. A heatsink fabrication method comprising the steps: a)forming a thin diamond film layer on a surface of a porous body having alow thermal expansion coefficient, and b) filling a hole in said porousbody with Cu after said step of forming of said thin diamond film layer.10. The heatsink fabrication method according to claim 9, wherein saidporous body comprises a sintered compact selected from the groupconsisting of a W sintered compact and a W—Mo sintered compact.
 11. Theheatsink fabrication method according to claim 9, wherein said porousbody has a porosity of at least 15% by volume and not more than 60% byvolume.
 12. The heatsink fabrication method according to claim 9,wherein said step of filling said hole in said porous body with Cuincludes permeating molten Cu into said hole in said porous body. 13.The heatsink fabrication method according to claim 9, wherein said stepof filling said hole in said porous body with Cu includes placing saidporous body onto a solid Cu and then heating and melting said solid Cuto permeate molten Cu into said hole.
 14. The heatsink fabricationmethod according to claim 9, wherein said step of filling said hole insaid porous body with Cu includes placing a solid Cu onto said porousbody where said thin diamond film layer is formed, and then heating andmelting said solid Cu to permeate molten Cu into said hole.
 15. Theheatsink fabrication method according to claim 9, wherein said step offilling said hole in said porous body with Cu includes storing molten Cuin a container, and permeating said molten Cu into said hole byimmersing said porous body where said thin diamond film layer is formedinto said molten Cu.
 16. The heatsink fabrication method according toclaim 9, further comprising scratching said surface of said porous bodyusing diamond, prior to said step of forming said thin diamond filmlayer.